This application claims the benefit of Korean Patent Application No. 2001-88604, filed on Dec. 29, 2001, which is hereby incorporated by reference for all purposes as if fully set forth herein.
1. Field of the Invention
The present invention relates to an organic electro luminescent display device, and more particularly to an organic electro luminescent display device which is able to achieve a low level of electric power consumption.
2. Discussion of the Related Art
Presently, there is a high demand for display devices to keep up with the high rate of growth of an information technology based society. Currently, one hundred million (100,000,000) cathode ray tubes (CRT) are required globally per year as display devices for desktop computers. Liquid crystal display (LCD) devices are used in notebook computers and can be applied to monitors, digital cameras and the like. An LCD is a non-emitting device and the image is displayed by a back light, while the CRT and electro-luminescence (EL) device are self-luminescent display devices. For example, the EL device can be divided into an inorganic EL device or an organic EL device depending on the fluorescent compound used.
The inorganic EL device can be classified as a distributed type, a thin film type, and an inorganic EL device. The inorganic EL device is operated by alternating current (AC), and the brightness of the device is dependent on voltage and frequency used.
The organic EL device has many advantages over LCD devices including a larger viewing angle, higher contrast, and superior visibility due to the self-luminescent characteristics. Additionally, because the organic EL device does not require a back light, it can take a thinner and lighter form than a LCD device, and it has lower electric consumption than the LCD. While the back light of the LCD must be on the entire surface, regardless of the displayed contents, the organic EL device is able to transmit current only to the pixels that need to be lighted. The EL device can be operated by low voltage direct current (DC) and is able to display moving pictures easily as it has a fast response speed. Accordingly, the organic EL device is being highlighted as the display for IMT-2000 standard. The organic EL device also has a wider temperature range of usage and is more resistant to vibration than the LCD device.
In the above organic EL device, positive and negative electrodes are generally formed on a transparent substrate, for example, glass, facing each other with an organic emitting layer formed therebetween. Light is emitted from the organic emitting layer by a voltage applied between the positive and negative electrodes. The positive electrode is formed by sputtering an indium-tin-oxide (ITO) thin film having high electric conductivity and light transmittance. Accordingly, light emitted from the organic emitting layer can be transmitted smoothly. The negative electrode is formed using a metal having a low work function, thereby applying the electrons smoothly.
Therefore, when the alternating (+) and (xe2x88x92) voltages are applied to the positive electrode and to the negative electrode, respectively, holes are injected from the positive electrode and electrons are injected from the negative electrode and combined in the organic emitting layer to emit the light. Additionally, the organic emitting layer comprises a hole transport layer, an emitting layer, and an electron transport layer.
In the organic EL display device, unit pixels are disposed in a matrix form. In addition, organic emitting layers of the unit pixels are driven selectively through thin film transistors disposed on respective unit pixels to display an image.
Hereinafter, the organic EL device having the above characteristics will be described in detail.
FIG. 1 is a view showing an equivalent circuit of the organic EL device having unit pixels with two thin film transistors disposed in a matrix form.
The unit pixel of the organic EL device, as shown in enlarged area A, comprises an Nth line of gate scan line (Gn) for supplying gate signals, an Mth column of data line (Dm) for supplying data signals, an Mth column of power voltage line (Pm) for supplying power voltage from one power voltage supplying line P, and first and second thin film transistors 10 and 20 formed on an area defined by the Gn, Dm, and Pm.
At that time, the gate scan line (Gn) and the data line (Dm) vertically cross each other, and an organic luminescence device 30 and the first and second thin film transistors 10 and 20 for driving the organic luminescence device 30 are disposed around the crossing point of the Gn and Dm.
The first thin film transistor 10 includes a source electrode 12 for receiving a data signal by an electrical connection to the gate scan line Gn. A drain electrode 13 is connected to a gate electrode of the second thin film transistor 20 for switching the organic luminescence device 30.
Additionally, the second thin film transistor 20 comprises a gate electrode 21 connected to the drain electrode 13 of the first thin film transistor 10. A drain electrode 22 is connected to a positive electrode of the organic luminescence device 30 and a source electrode 23 is connected to the power voltage line (Pm). Therefore, the second thin film transistor 20 functions as a transistor for driving the organic luminescence device 30.
Although, not shown in detail in FIG. 1, the organic luminescence device 30 comprises a positive electrode (+) connected to the drain electrode 22 of the second thin film transistor 20. A negative electrode (xe2x88x92) is connected to a common electrode and an organic emitting layer 31 formed by being inserted between the positive electrode (+) and the negative electrode (xe2x88x92). Additionally, the organic emitting layer 31 comprises a hole transport layer, an emitting layer, and an electron transport layer.
Further, the organic luminescence device 30, comprising a capacitor having one electrode is connected to the power voltage line (Pm). The other electrode is connected to the drain electrode 13 of the first thin film transistor 10 and to the gate electrode 21 of the second thin film transistor 20, commonly. The power voltage line (Pm) is connected to the power voltage supplying line P, disposed on the edge of the panel. The power voltage is supplied to respective pixels by the power supplying lines, which are divided from one power supplying line regardless of emitted color on the organic luminescence device.
The power voltages required by the respective pixels are different for the various desired emitted colors of the organic luminescence device. That is, the operating voltage needed to radiate a blue color luminescence device is different from the operating voltage for radiating a red color luminescence device. Additionally, the operating voltage for emitting a green color luminescence device is also different. For example, the required operating voltages are in order of blue (B) greater than red (R) greater than green (G).
For example, the power voltage is applied to all colors of devices. The operating voltage of the blue luminescence device has the highest operating voltage and one power voltage supplying line and a common electrode as in the related art. There are voltage differences between the applied power voltage and the voltages required to operate the G pixel and the R pixel which can be operated by small applied voltages.
In addition, the voltage difference between the operating voltage and the source voltage is a principal cause of electric power consumption increase.
Accordingly, the present invention is directed to an organic electro luminescent display device that substantially obviates one or more of the problems due to the limitations and disadvantages of the related art.
An advantage of the present invention is to reduce the amount of electric power used in the panel of an organic luminescence device. This may be accomplished, for example, by setting a power voltage supplying line or a common electrode individually on R, G, and B pixels. Accordingly, the appropriate operating voltages can be supplied to the respective pixels.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, there is provided an organic electro luminescent display device which includes a gate scan line and a data line, wherein the data line and the gate scan line cross. Red (R), green (G), and blue (B) pixels are arranged in a matrix form in an area where the gate scan line and the data line cross. An organic luminescence device corresponding to the R, G, and B pixels for emitting R, G, and B colors by an electric field applied to a positive (+) and negative (xe2x88x92) electrodes is provided. A switching unit for switching image information applied from the data line by a scan signal applied from the gate scan line and a driving unit for applying the electric field to the organic luminescence device according to an image information applied through the switching unit are provided. A power voltage supplying line formed individually on the R, G and B pixels for applying different power voltages to the driving units is formed on the respective pixels. A common electrode for supplying a common voltage to the organic luminescence device is provided.
The organic luminescence device supplies only the required operating voltages to the respective R, G, and B pixels. Accordingly, power consumption advantages are present.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.